1. Field of the Invention
High optical quality (high-Q) optical resonators, including whispering gallery mode (WGM) microresonators, have been the subject of investigation because of their strong potential for use in high-performance photonic devices. The Q-value, Q or Q-factor (quality factor) of a resonator is a measure of its energy storage capacity and the internal buildup of optical fields, and is reflected by the linewidths of the optical resonances in the microresonator. Potential applications for these devices include ultra-sensitive molecular detectors as well as advanced light sources, such as narrow-linewidth lasers and comb generators. The unique characteristics of these devices are particularly relevant for mid-infrared (MIR) applications, because of the stronger molecular absorption lines in the MIR, and because of the increasing need for frequency comb sources in this “molecular fingerprint” region.
Glasses represent an excellent class of materials for the fabrication of high-Q optical resonators, because of the relative ease of fabricating high-purity glasses with ultralow absorption losses and their amenability for high-concentration doping of rare-earths, as needed for luminescent sources and lasers. Glasses may easily be formed into optical resonators by fabrication processes such as melting and cooling into microspheres. Success has been achieved in the fabrication of optical resonators based on silica glasses, including demonstration of microspheres with Q's as high as 1010 at near-IR wavelengths. However, due to the rapid increase in multi-phonon absorption at wavelengths>2 microns, silica glasses are not usable for fabrication of high-Q MIR optical resonators and are limited to applications in the near-IR spectral range.
Potential low-phonon energy glasses developed for MIR applications include fluorides, chalcogenides, and tellurides. The basic physics of microsphere formation in glasses is similar to the formation of macrospheres such as marbles or ball bearings. It involves a relatively well-understood interplay between the surface tension and viscosity of the material. However, the need for sufficiently high Q's (>107) in optimal microspherical optical resonators imposes severe demands on the formation process, notably on the shape and smoothness of the surface, and on the minimization of subsurface structural imperfections, caused in part by the inevitable formation of microcrystallites in glasses. Thus, fabrication of high-Q optical resonators from MIR glasses such as ZrF4—BaF2—LaF3—AlF3—NaF (ZBLAN) and indium fluoride (InF3) as well as tellurides and chalcogenides presents challenges not found in processes concerning silica glasses. Fabrication of optical resonators from these materials requires a better understanding of the microsphere formation process, and more precise control of the melting and cooling processes, and of the ambient environment.
Previous efforts to fabricate optical resonators from MIR glasses based on conventional methods including microwave plasma heating and CO2 laser heating used for the fabrication of silica microspheres have resulted in microspheres of poor surface quality (and therefore low Q's) due to significant differences between the physical and thermo-optical characteristics of MIR glasses and silica. More specifically, the small temperature difference between the melting and crystallization temperatures (Tm and Tx) and—more importantly—between the glass softening and crystallization transition temperatures (Tg and Tx) facilitates the creation of highly scattering microcrystallites that degrade the Q-factor of the microsphere due to bulk and surface scattering. The value of (Tg−Tx) and the consequent glass stability is dependent on the heating rate and precise control of the local temperature and cylindrical symmetry of the heating source, with slower heating rates leading to larger values of (Tg−Tx), and thus to reduced crystallization.
The largest reported Q-value of a ZBLAN microsphere is about 106. This was achieved via the use of free fall techniques with or without zero gravity environments and with or without the use of liquid “catch basins” for the falling microspheres. In these experiments, free falling Er: ZBLAN microparticles of a large range of uncontrolled sizes, formed from ground powders, were melted by large heaters—such as a microwave plasma torch—during the free fall, and surface tension resulted in the formation of microspheres. Microspheres have also been fabricated in chalcogenide glasses using similar techniques, and asymmetric (non-cylindrically symmetric) electric heaters. In general, the above described methods are complicated and impractical, and result in low yields and poor size control.